A metal-halide lamp converts into radiation the power dissipated by an electric current passing through a gaseous medium at greater than atmospheric pressure. Appropriate selection of the gaseous medium provides favorable spectral distributions of radiated power. As a result, a metal-halide lamp is substantially more efficient than an incandescent lamp.
A typical double-enveloped metal-halide lamp comprises an inner arc discharge tube containing high-pressure gas or vapor including mercury, metal-halide additives, and a rare gas to facilitate starting. The arc tube is enclosed in a hermetically sealed outer envelope or jacket. The outer envelope is filled with nitrogen or another gas or atmosphere which is inert with respect to internal lamp parts. The arc tube is fabricated from quartz or fused silica, and the outer envelope is formed from a hard glass, such as borosilicate glass. The outer envelope provides thermal insulation, protection of arc tube seals from oxidation, and absorption of short wavelength ultraviolet rays emitted from the arc tube. See, for example, U.S. Pat. No. 3,407,327, issued Oct. 22, 1968, to Koury et al.
One design factor associated with a metal-halide lamp is heat loss from the arc tube by means of convective currents within the atmosphere of the outer envelope. Convective heat loss is caused by transporting heat from the arc tube to the outer envelope by means of gaseous convection currents in the atmosphere within the outer envelope. It is generally true that the overall efficiency of a metal-halide lamp is improved with higher operating temperature of the arc tube walls. Higher operating temperature causes greater quantities of the metal-halide additives to be in the vapor state. An excess of additives is usually provided to insure a saturated vapor state within the arc tube. With more vaporized additives, the luminous output and color temperature of the lamp are improved (i.e., lower color temperature) in most cases. Therefore, it is important to keep heat lost via convection at a minimum. In regard to convective heat loss, a vacuum in the outer envelope is desirable since convective flow would be eliminated.
Another design factor associated with a metal-halide lamp is the problem of sodium loss. Most metal-halide lamps contain a sodium compound as one ingredient of the arc tube fill. During the life of the lamp, sodium migrates through the walls of the arc tube thereby adversely affecting lamp performance. One proposed explanation of the process by which sodium loss occurs is as follows. During operation of the lamp, a photoelectric process, caused by the flux of ultraviolet radiation emitted from the arc tube and incident upon the metal frame parts, liberates electrons which migrate to and collect on the arc tube. The electrons on the outside of the arc tube create an electric field which draws sodium ions through the arc tube walls into the atmosphere of the outer envelope. This process depletes the sodium from within the arc tube causing diminished luminous efficacy and maintenance and, ultimately, reduced lamp life. For a more detailed explanation of the sodium loss problem, see Electric Discharge Lamps, by John F. Waymouth, The M.I.T. Press, 1971, Chapter 10, and further references cited therein.
From the viewpoint of sodium loss, a gaseous fill at a substantial pressure within the outer envelope is desirable. The presence of gas molecules of the fill impedes the migration of sodium ions from the outer surface of the arc tube to the metal frame parts within the outer envelope. Increasing the fill pressure increases the density of gas molecules and thereby reduces sodium loss.
Yet another design factor associated with a metal-halide lamp is the possibility of striking an electrical arc between the lead-in wires inside the outer envelope. This "arc-over" problem is especially significant when the atmosphere of the outer envelope is at low pressure, e.g., less than 10 torr. For a more detailed explanation of the arc-over problem, including typical Paschen curves showing ignition potential as a function of fill pressure for various gases, see Light Sources, by W. Elenbass, Crane, Russak & Co., Inc., New York, 1972. Regarding the possibility of arc over, a gaseous fill within the outer envelope at a substantial pressure is desirable.
In the event the outer envelope of a metal-halide lamp should be fractured for any reason, the implosion forces will be minimized when the pressure within the outer envelope is as close as possible to the external atmospheric pressure. Regarding this safety factor, a gaseous fill within the outer envelope at the same pressure as the external atmosphere is desirable.
There is another safety consideration associated with the design of a metal-halide lamp. There is a small probability that an arc tube may burst during lamp operation. In the rare event of an arc tube burst, it is highly desirable that the outer envelope of the lamp remain intact. To this end, some sort of burst-restraint structure between the arc tube and outer envelope is desirable. Naturally, such burst-restraint structure should have minimal effect on lamp performance. For examples of various burst-restraint structures, or containment devices, see U.S. Pat. No. 4,888,517, issued Dec. 19, 1989, to Karlotski et al.
The foregoing, while not a complete enumeration of design factors, nevertheless points out some of the conflicting objectives facing a metal-halide lamp designer particularly with respect to the design of the atmosphere within the outer envelope. A vacuum within the outer envelope is desirable for heat insulation of the arc tube and the concomitant improvements in color temperature and luminous efficacy while a gaseous fill at a substantial pressure is desirable for minimizing sodium loss and the likelihood of arc over.
In U.S. Pat. No. 3,619,682, issued Nov. 9, 1971, to Lo et al., there is disclosed a high-wattage double-enveloped metal-halide lamp including means for forcibly cooling the outer (second) envelope. This patent suggests a container or third envelope surrounding the lamp. The container cannot be sealed. It necessarily includes an inlet and outlet for circulating a suitable coolant so that the outer envelope may be forcibly cooled. Moreover, the space between the arc tube and second envelope necessarily must be filled with a fluid which has adequate heat-transfer properties. The overall teaching of Lo et al. is to facilitate heat dissipation from the arc tube and not to conserve heat from the arc tube.
In Fohl et al., U.S. Pat. No. 4,499,396, issued Feb. 12, 1985, there is disclosed a double-enveloped metal-halide lamp having a convection-suppressing enclosure surrounding the arc tube. The enclosure may be closed on both ends. There is no teaching that the enclosure may be hermetically sealed with a vacuum on the inside. The patent teaches that the Rayleigh Number in the region laterally surrounding the arc tube within the enclosure must be controlled in order to limit convective heat loss in this region. The need to suppress convective heat loss in the region presupposes an atmosphere other than a vacuum within the enclosure.
In U.S. Pat. No. 4,791,334, issued Dec. 13, 1988, to Keeffe et al., there is disclosed a double-enveloped metal-halide lamp having a heat-redistribution enclosure surrounding the arc tube. The enclosure may be closed on both ends. The atmosphere within the outer envelope is a vacuum. There is no teaching that the enclosure may be hermetically sealed nor that the atmosphere within the enclosure may differ from the atmosphere within the outer envelope.
These prior art examples illustrate that a metal-halide lamp having the advantages of a vacuum within the outer envelope is known and a lamp having the advantages of a gaseous fill within the outer envelope is known, but there appears to be no prior art example of a single lamp having the combined advantages of a vacuum and a gaseous fill within the outer envelope. In the prior art, these differing advantages appear to be mutually exclusive in the sense that either one set of advantages or the other set is attainable but not both sets in the same lamp.